added fallback routine for finding chromosomes in sub get_chromosomes in cases where there are no chromosome features in the dataset.
added fallback routine for finding chromosomes in sub get_chromosomes in cases where there are no chromosome features in the dataset.

asherkhb/coge,LyonsLab/coge,LyonsLab/coge,asherkhb/coge,asherkhb/coge,asherkhb/coge,LyonsLab/coge,LyonsLab/coge,LyonsLab/coge,LyonsLab/coge,asherkhb/coge,asherkhb/coge,LyonsLab/coge,asherkhb/coge,LyonsLab/coge,asherkhb/coge,asherkhb/coge,LyonsLab/coge